The Characteristics and Oxidation of Vapor - Liquid - Solid Grown Si Nanowires

Abstract

The Vapor - Liquid - Solid (VLS ) technique allows the growth of high aspect ratio Si wires. The Si nanowires formed by this technique can be thinned down by oxidation. This approach allows the formation of very thin Si cores which may be used to research the properties of Si nanostructures. In this work the growth and oxidation of these wires is characterized.In the growth a very thin layer of Au is deposited on a Si (111) surface, silane gas is introduced into the chamber as the Si source gas and the temperature is raised to 300 – 600°C. Initially a catalytically active Au surface phase leads to the growth of a defective epitaxial Si layer. As Au / Si molten alloy balls nucleate and grow in size to approach the threshold size for VLS wire growth, which is determined by the Gibbs - Thomson effect, the epitaxial layer growth rate decreases and a transition to Si nanowire growth occurs. The morphology and width of the wires is strongly dependent on the growth temperature and pressure. At low pressure and high temperature relatively thick well-formed wires grow straight up from the substrate surface along the [111] direction. As the temperature is decreased and the pressure is increased thinner wires (as thin as 10 nm ) grow which tend to exhibit growth defects. A light oxidation yields Si cores which are of the order of 5 nm in diameter.

Growth, Thermodynamics, and Electrical Properties of Silicon Nanowires

“…27 A related, ubiquitous problem is that a certain percentage of the nanowires tends to change their growth direction during growth; as a result, they show a kink (see Figure 1b). 29,45,46 This kinking problem, however, can be circumvented by growing the nanowires inside a template such as anodic aluminum oxide (AAO). [47][48][49] The template forces the nanowire to grow straight along the pore direction.…”

Si-containing precursors for Si-based anode materials of Li-ion batteries: A review

“…49,52 The gaseous silicon-bearing precursors, such as silane (SiH4), disilane (Si2H6), dichlorosilane (SiH2Cl2), and tetrachlorosilane (SiCl4), are always employed as the silicon sources for synthesizing silicon nanowires, and gold (Au) is the most popular metallic catalyst for the VLS growth of nanosized silicon. [77][78][79][80][81] The name "VLS mechanism" refers, of course, to the fact that silicon from the vapor passes through a liquid droplet and finally ends up as a solid. 75 Briefly, this process involves the following stages: 1) formation of the liquid droplets of the metal-silicon alloy on the surface of the substrate; 2) the dissolution and diffusion of gaseous-silicon-based precursor into the silicon-Au alloy droplets; and 3) silicon precipitation and axial crystal growth due to supersaturation and nucleation at the liquid/ solid interface.…”

Facile pyrolytic synthesis of silicon nanowires

“…A common perception is that the size of catalyst/silicon droplet controls the dimension of the NW. Although intuitively appealing, the results of Givargizov [42], Westwater [23], and Sunkara [28] suggest that the degree of silicon super-saturation in the eutectic catalyst may play an even bigger role. We studied the SiNW diameter by systematically varying the silicon precursor concentration in reaction glass tube, thereby modulating the degree of supersaturation in the AuSi eutectic nanodroplet.…”

“…Standard synthesis methods include chemical vapor deposition [22,23], laser ablation [24][25][26], microwave [27,28] or oxide assisted growth [1,[29][30][31][32][33][34]. Such methods often lead to small yields of SiNWs, unfortunately accompanied by variable thickness of native oxide capping them.…”

“…The VLS mechanism is generally controlled by the synthesis/growth temperatures (i.e., corresponding to eutectic temperatures and above for a given metal-silicon combination) and the terminal location of the metal catalyst at the growing tip of SiNW. Givargizov [42] and Westwater [23] et al further clarified the elemental steps involved in the growth process and have concluded that the super-saturation of the Si-catalyst controls the effective diameter of the Si-NW. In cases of rapid growth, e.g., laser ablation, the size of the catalyst is believed to control the diameter of the SiNW [43].…”